What @HDMIGuy said was for Roku TV devices, and is spot on.
However, if you have a different TV (doesn't matter if it's a smart TV or not), you would watch cable like always. You'd connect your Roku device to an HDMI port on the TV and switch the input to that HDMI port when you wanted to watch Roku. Depending on the capabilities of the TV, it may automatically switch when you hit the HOME button on the Roku remote.
But using a Roku device and still have cable? Sure.
DBDukes Roku Community Streaming Expert Note: I am not a Roku employee.
If this post solves your problem please help others find this answer and click "Accept as Solution."